Such mass flow meters are known to have at least one measuring tube which is excited to mechanical vibrations, is flowed through by a fluid to be measured and can be bent or straight; details on this are specified below in conjunction with the explanation of FIG. 1.
Usually, at least one vibration exciter as well at least two vibration sensors are arranged on the measuring tube, the latter spaced apart from one another in the flow direction. The measuring tube mostly vibrates at a mechanical resonant frequency which is prescribed by its material and its dimensions but is varied by the density of the fluid. In other cases, the vibration frequency of the measuring tube is not exactly at the mechanical resonant frequency of the latter but in the vicinity of said frequency.
The vibration sensors generate analog sensor signals whose frequency is equal to the vibration frequency of the measuring tube and which are mutually phase-shifted. A measuring subcircuit fed by the sensor signals supplies a signal proportional to the mass flow rate, and a exciting subcircuit feeds the vibration exciter with alternating energy whose frequency is usually equal to the instantaneous vibration frequency of the measuring tube.
U.S. Pat. No. 4,801,897 describes a exciting subcircuit which is in the manner of an analog phase-locked loop, that is to say the frequency of the alternating energy adjusts itself automatically to the instantaneous mechanical resonant frequency varied by the density of the fluid.
Conventional measuring subcircuits are either circuits which are analog and thus operate in the time domain such as, for example, the measuring circuit described in EP-A 698 783, which corresponds to the U.S. application Ser. No. 08/514,914 of Aug. 14, 1995, or in U.S. Pat. No. 4,895,030, or digital circuits such as, for example, the measuring circuit described in U.S. Pat. Nos. 4,934,196, 4,996,871, 5,052,231, 5,429,002 or in EP-A 702 212.
Concerning the measuring circuit described in EP-A 698 783, the only point of interest for the invention--because it has recourse thereto--is that, inter alia, it contains an analog control circuit which controls the sensor signals to equal amplitudes.
The measuring circuit described in U.S. Pat. No. 4,895,030 subjects a sum signal formed from the two sensor signals and a difference signal formed from the two sensor signals to separate analog filtering based on an analog Fourier transformation, and then forms the signal proportional to the mass flow rate from the signals thus filtered.
The measuring circuit described in U.S. Pat. No. 5,052,231 subjects each sensor signal to amplification, then to respective anti-alias filtering, thereupon to respective amplifying sampling/holding, thereupon to respective analog-to-digital conversion and finally to respective discrete Fourier transformation. A digital signal proportional to the mass flow rate is calculated from the digital signals thus formed in accordance with a specific algorithm by means of a microprocessor. In this processing, the frequency of a sampling signal controlling the abovementioned sampling/holding must once again be equal to an integral multiple of the mechanical resonant frequency of the measuring tube.
The measuring circuit described in U.S. Pat. No. 5,429,002 subjects each sensor signal to sampling/holding, thereafter to respective analog-to-digital conversion and, finally, to respective digital processing which realizes the principle of the least-squares sine fit.
According to the measuring circuit described in EP-A 702 212, each sensor signal and a sum signal formed therefrom are subjected to respective anti-alias filtering, thereafter to respective analog-to-digital conversion, thereafter to respective bandpass filtering and, finally, to respective discrete Fourier transformation.
A digital signal proportional to the mass flow rate is calculated according to a specific algorithm from the digital signals thus formed. Here, as well, the frequency of a sampling signal controlling the abovementioned sampling/holding must again be equal to an integral multiple of the mechanical resonant frequency of the measuring tube.
In the measuring circuits so far referred to, it has been more or less tacitly assumed that the information on the value of the instantaneous resonant frequency of the measuring tube, that is to say essentially the information on its numerical value, is always available on the analog side of the circuit or can be determined easily on this side, so that the value of this variable can be, if necessary, incorporated into the formation of the signal or digital signal proportional to the mass flow rate. Thus, the instantaneous resonant frequency can be determined, for example in the arrangement according to the abovementioned U.S. Pat. No. 5,429,002, by means of a zero-crossing detector.
However, this is not the case in the measuring circuits described in U.S. Pat. Nos. 4,934,136 and 4,996,871, rather, this frequency information is generated only on the digital side. In these measuring circuits, each sensor signal is subjected to anti-alias filtering, thereafter to respective sampling/holding, thereafter to analog-to-digital conversion and, finally, to respective discrete Fourier transformation.
A digital signal proportional to the mass flow rate is calculated according to a specific algorithm from the digital signals thus formed by means of a microprocessor. The frequency of a sampling signal controlling the abovementioned sampling/holding is--as previously mentioned--no longer equal to an integral multiple of the mechanical resonant frequency of the measuring tube, but is selected arbitrarily.
The information on the instantaneous value of the mechanical resonant frequency of the measuring tube is obtained by virtue of the fact that the microprocessor determines the maximum of the power spectrum of the Fourier-transformed digital signals, and the frequency belonging to this maximum, which is equal to the resonant frequency.
Although said U.S. Pat. Nos. 4,934,196 and 4,996,871 describe a Coriolis-type mass flow meter with extensive digital circuits, only an analog exciting subcircuit is then explained with the aid of their respective FIG. 4.